No, to stay explicitly with your question, no way can they tell you are tethering.... as it is just DATA.
I used FoxFi for several years and never had a problem with VZW yelling at me that I was not "buying their tether plan".
Want to make it harder for them?
install a VPN plan on your phone. I have
www.purevpn.com installed on my phone when I want to "disappear"....
I always use VPN when I use public WiFi routers. No snoop dogs are going to catch me entering any passwords. My IP changes all over the place every time I sign on.... depends on my mood.
I'm "in Jacksonville, Florida" as I type this.
Yesterday I was in Mexico City
When you login to pure vpn, you can select the server you want, or just accept the Worlds Fastest Server. Your choice.
I bought the annual package, and paid up front, it comes out to $4.16/month which I think is very reasonable. They allow you to install their vpn on 5 devices in your household for that same price.
Odd thing is, on my desktop PC, World's Faster Server last week was Mexico City and my download speeds according to
www.speedtest.net were a little over double my normal AT&T ADSL speed limted plan.
On my cellphone, it is both faster and slower, depends on time of day I guess.